Gain control apparatus



March 15, 1932. w. H. T. HOLDEN 1,849,189

GAIN CONTROL APPARATUS Filed Dec. 16, V1950 LEN INVENTOR WEZ'IM ZWW ATTORNEY Patented Mar. 15; 1932 marten {star-Es PATENT QFFICB WI'ELIAM Hi T'. HOLDEN; OF BROOKLYN, NEW YORK, ASSIGNOIR; T0 AMERICAN TELE- PHONEA'ND" COMPANY, A CORPORATION OF NEW YORK GAIN-CONTROL APPARATUS Application filed December 16,: 1930. Serial No. 502523,

This invention. relates" to signaling systems. More particularlygithis invention relates to apparatus for the. control of the level of signaling energy.

One of the objects of thistinvention is to automatically control the; gain of a signal translating system. Another of the objects is to maintaim the level of signaling energy constant in value tor variable within very narrow limits.

This application disclosesnovel arrangements for automatically controlling the gain of a signaling system embodying features which are'believed to have many advantages over other priorarrangements for gaincontroli The arrangements of this invention re nice no: ungrou nded batter'yf suppliesfother" t an dry ce'lls fbrgridbias, an'd the elements are: sc -"set" up as not? to; causeany modulation or" distortion of the signals: or'currents'; the

gailraofwhichathe arrangements are intended ton controli G011'sequtentl y,this invention is adaptable to: voice frequency circuits; or to V multi ehannel carrier amplifiersor othersys terns. No movingpartsgarerequired;

'- This invention willbe hetter understood from the detailed description?hereinatterfoh lowing; when read in connection .W-ithfthe' acv company-ing'drawings,in WhichFigure 1' rep'--' resents one embodiment of? this inventionim. cludinga Tr-type networkihavingequal series 1 elements andia: shunt element? formed by the r plate and-efiianientl' electrodes of-t a vacuum P tubeetheliinpedance ofiwliicltis variedibycurrenttderivedi from v a' rectifienuand' Fig. 2' discloses a similar." network embodying three:

vacuum :tub'e's two forming: serieselements andl the'zthirdrai shunt: element;.al1 automati- 1 callyvadjustahlea in! impedance;-v V Referring; to: Fig?- 1 of the drawings; the vacuum. tubes: A ai'lCl A'g form: two tubes of an amplifying; system, t-liegain of Which is to be'automatically controlled. This amplify h ing;system-transmitssignaling currents from' aninput circuitK toran output-circuit K Vacuum-tubes D and .V are controlling devices which are"interposedfbetween the input.

circuitKf and the network coupli ng the am- L plifyiiig tubesA anclA as Will be described in greater detailhereinatter. Re'sistances R andR and the plate impedanceo'fthe tube form the essential elements of a T network Which is preferably one of a rather high impedance for example about 50,000 ohms, this being the eflective impedance oft-he coupling arrangement vvhen the tube V exhibits its maximum plateimpedance;

The primary Windin'gof the-transformer T is connectedto the input circuit K The secondaryvvindingof this transformer is con nected to the fixed terminals of ja potenti'om eter P, the movable arm ofwhi ch is connected to the grid electrode of the tube D vvhichis one: of the heater type and is employed to act as arectifier. The lowerterminal of the potentubeuD through a battery B 1 The filament tiometer P is connectedlto the heater of the V of the tube Dis heated by a battery B The V heater of the tube D is connected to ground through a'variable resistance R The plate electrode of the tube D is connectedto ground:

througha. battery ]3 and an inductance 11 This electrode is also connected toground through a condenser C The lower terminal of the potentiometer P'is connected to grid electrode otth'e tube V", and a condenser C is interposed betweenthis electrode and the grounded filament of the tube V. The plate electrode of thetube- V is connected to the positive terminal of the battery B through 'an inductance L The plate electrode of the tube V is also connected to the terminal common to resistances iii and R through condenser C \VlllCll prevents the potential of the battery B from reaching re-v sistances R and R and the Windings'of the tra'nstoriners connected thereto. The fila-' ment of the tube V is heated to incandescence by a battery B The potential or the grid electrode of the tube D is determined by 'the battery B as Well ashy the voltage impressed'by the input a circuit K through transformer Ti and'the potentiometer P. This potentiometer Will determine the amplitude of the current rectifi'ed by'thetube D, and it may beset so as to produce any fixed direct current for an input of any predetermined magnitude. The plate circuit of the tube D may be considered to include the battery B inductanceL the plate electrode and the heater of tube D, the resistance R and ground. The plate circuit of the tube V may be considered to include the battery 13,, inductance L the plate and filament electrodes of the tube V, and ground. The plate and filament electrodes of the tube V are the essential elements forming the shunt impedance of the coupling T network of this invention, and it will be shown that this shunt impedance is variable in accordance with the level of signaling energy flowing in the circuit K The battery B is of such a potential that it will render the grid electrode of the tube D sufficiently negative with respect to the heater that substantially no current will fiow through the plate circuit of the tube D, especially if the input circuit K impresses no voltage upon the tube D. Battery 13 is, however, common to the grid electrode of the tube V, the circuit interconnecting the grid electrode of the tube V with its filament electrode including battery 13,, resistance R and ground. For a fixed input to tube D, there will be a direct current of predetermined magnitude in the plate circuit of the tube D. lnductance L and a condenser form a filter which acts to suppress any superimposed alternating current and, obviously, this filter may include further sections including series inductive elements and shunt capacitive elements.

As the input to tube D changes, the direct current flowing through the plate circuit of.

the tube D will correspondingly vary. This will similarly change the potential impressed across the terminals of the resistance R As the current in the plate circuit of the tube D increases, the drop in potential across the resistance R, will increase, and since the potential impressed across the latter resistance acts in a direction opposite that of the'battery 13 so far as tube V is concerned, the impedance between the plate and filament electrodes of the tube V will be lowered. Consequently, the loss introduced by the T network between tubes A and A will be increased.

When the input to tube D is very small, there will be substantially no current in the plate circuit of the tube D and the potential impressed across the resistance R, will be very small. Therefore, the impedance between the plate and filament electrodes of the tube V will be very high, and the loss introduced by the T network coupling tubes A, and A will also be very low.

Thus, it will be seen that the coupling network of this invention varies the loss introduced between a pair of amplifying tubes as the shunt impedance of the tube V is varied. The surge impedance of this network will vary, but inasmuch as this is arranged in the inter-stage circuit between the two amplifying tubes, it will not introduce any reflection Shunt Total loss resistance in decibels 1,000 40.0 3,000- 32.0 5,000 26.0 8,000 23.2

co 6.0 0 co It will be seen that the variations in the plate resistance of the tube V causing a change from 3,000to 85,000 ohms, will vary the gain of the amplifier by about 14.8 decibels.

It is to be noted that the plate impedance of the tube V is varied by changing the direct current bias of its grid electrode. This is accomplished by the novel employment of the equi-potential tube D acting as a rectifier. The plate current of this rectifying tube rises as the input thereto increases, and correspondingly changes the potential impressed across the resistance R Obviously, the effectiveness of the gain control. arrangement of Fig. lcan be varied by changing the input to the rectifier D by means of the potentiometer P or changing the resistance R The rapidity with which this arrangement will readjust gain is determined by the cutoff frequency of the filter L -C or by the time constant of the circuit B C whichever is the lower.

In carrier frequency systems where the input to the rectifier D is a pilot current, in which case the transformer T would'be tuned as, for example, by placing a variable condenser in series with its primary winding and/ or a variable condenser in shunt with its secondary winding, the arrangement shown can be allowed to operate quite rapidly, as it would be impracticable to attempt to slow down the action of the device so as to disregard ordinary line swings. For voice frequency circuits, the condenser C would be about one microfarad, especially since the'reh oaths and one shunt s ace dischar e athp The reference characters 7 and V represent the amplifying tubes and these are interposed between the input circuit K and the output circuit K in order to amplify the transmitted signals. Tubes V V and V represent three tubes, all of the heater type, forming the essential elements of the T network, tubes V and V forming the series elements and tube v the shunt element. It will be shown that these tubes provide impedances which simultaneously vary, the impedances of the tubes V and V varying by equal amounts in the same direction. 1

Part of the energy in the output circuit K is transmitted to an amplifier V through transformer T and potentiometer P. The amplified energy is then transmitted through transformer T to a rectifying tube V, which is also of the heater type. The rectifying circuit includes the secondary winding of the transformer T the plate electrode and the heater of the tube V a winding L and resistances R and R Winding L and condensers C and C form a section of a wave filter which is intended to suppress low frequency alternating currents.

The heaters of the tubes V V and V are grounded in common through a winding L The grid electrodes of the tubes V and V are connected to the lower terminal of the resistance R through a winding L These electrodes are both grounded through a condensor C. The upper terminal of the resistance R is connected to the gridelectrode of the tube V. through a winding L and a grid biasing battery B The terminal common to the resistances R and R is grounded and the upper terminal of the winding L is grounded through a condenser C The plate electrodes of the tubes V and V; are tied together through windings L and L and the terminal common to these windings is grounded through a plate battery B The plate electrode of the tube his connected to ground through a winding L and a battery ages impressed thereacross willdilfer by: a corresponding amount. The voltage produced by resistance R will be impressed on the grid electrode of tube V, through battery B and winding L the voltage of battery B opposing that produced by the resistance R The voltage produced by the resistance R will be equally impressed onthe grid electrodes of the tubes V and V through the winding L 1 i As the current rectified by tube V 'increases above a predetermined value by virtue of an increase in the energy in the circuit K and impressed upon the amplifier V the voltages'across the resistances R and R will increase above certain normal values. Then, there will be a reduction in thebias on the grid electrode of the tube-V and, moreover, an increase in the bias'on the grid elec trode tubes V and V Hence, the loss introduced by the T network will, accordingly, be I increased. As the rectified current flowing through tube V is reduced below its normal, predetermined value, the voltages produced by resistances R and R will also be reduced below their normal values. Hence, there will be an increase in the bias of the grid electrode of the tube V and decreases in the bias of the grid electrodes of the tubes V and V 7 Accordingly, the loss introduced by the net- 7 work will be reduced. v V

Thus, it will be seen that the coupling apparatus'shown in Fig. 2 will maintain the output at the circuit K constant or variable within very narrow limits. The tubes V and V provide impedances which are the series elements of the coupling arrangement and the tube V provides the impedance forming the shunt element of the arrangement- All of these impedances are simultaneouslyvariable and, moreover, the terminating 'im-- pedances of the network are maintained highly constant under all conditions. Consequently, the'effectiveness of the control arrangement is vastly improved.

While this invention has been shown and described in certain particular arrangments merely for the purpose of illustration, it will be understood that this invention may be applied to other and widely varied organizations without departing from the spirit of the invention and the scope of the appended claims. 1

N hat is claimed is: p

1. In a gain control system for a pair of amplifiers, the combination of a circuit interconnecting said amplifiers, a pair of equal resistances connected inseries in said circuit, a tube having electrodes forming a space discharge path and a control electrode, said electrodes forming the space discharge path of said tube being connected in shunt across said circuit from the point common to said.

said tube in accordance with the energy transmitted through said amplifiers.

2. In a gain control system, the combination of a transmission circuit, three tubes each having two electrodes forming a space discharge path and a control electrode, the electrodes forming the space discharge paths of two of said tubes being connected in series with said transmission circuit, the electrodes forming the space discharge path of the third tube being connected in shunt with said transmission circuit .from the point common to the space discharge paths of the first two tubes, and means to simultaneously vary the potentials of the control electrodes of all of said tubes.

3. In a gain control system, acoupling network comprising three tubes each having two electrodes forming a space discharge path and a control electrode, the electrodes forming the space discharge paths of two of said tubes being connected in series relationship in said network, the electrodes forming the space discharge path of the third tube being connected in shunt in said network from the point common to said space discharge paths of the first two tubes, and means to simultaneously vary the potentials of the control electrodes of the first two tubes and the potential of the control electrode of the third tube in mutually opposite directions.

In testimony whereof, I have signed my name to this specification this 15th day of December, 1930.

WILLIAM H. T. HOLDEN. 

